According to drive current and back EMF waveform, a permanent magnet motor may be classified into a sine-wave permanent magnet motor and a square-wave permanent magnet motor. Generally, the sine-wave permanent magnet motor is referred to as a permanent magnet synchronous motor (PMSM) or a sine-wave AC servo motor, while the square-wave permanent magnet motor is referred to as a square-wave brushless DC motor (BLDCM).
The square-wave permanent magnet motor was widely used in 1980s. It has almost the same external characteristics as a brush DC motor and can be controlled in a simple mode. However, the biggest drawback of the square-wave permanent magnet motor is its relatively large switching torque fluctuation. Researchers have proposed many compensation measures, trying to overcome this drawback, but none of these measures has gained desired effects in applications.
Since the sine-wave permanent magnet motor has a much smaller moment fluctuation than the square-wave permanent magnet motor, it has gradually started to take the place of the square-wave permanent magnet motor since 1990s in accurate servo drive applications, and now become the mainstream of the current industrial applications. Nevertheless, accompanied by the use of the sine-wave permanent magnet motor is a much more complicated control system and a rapidly increased manufacture cost. What is more important, the force parameters of motor are significantly reduced.
On the other hand, the conventional square-wave brushless DC motors and the controlling technology thereof have been recognized as mature. Due to the aforesaid deficiency, it is restricted to applications that do not have high requirements. Little research has been conducted on the square-wave brushless DC motor ever since at home or abroad.
To solve the aforesaid technical problems, the inventor of the present invention filed earlier a patent application NO. CN101371425A for an invention titled “SQUARE-WAVE THREE-PHASE BRUSHLESS PERMANENT MAGNET DIRECT CURRENT MOTOR”, which provides a motor having a number 2P=8 of poles. Objectively speaking, this motor still suffers from a great iron loss in high speed applications.